[U-Boot] [RFC] sunxi: Support uploading 'boot.scr' to RAM over USB OTG in FEL mode

[Ian Campbell <ijc+uboot@hellion.org.uk>]

On Tue, 2015-02-24 at 04:48 +0200, Siarhei Siamashka wrote:
> In order to fully support booting the whole system over USB OTG
> without relying on anything else (MMC, SATA, USB sticks, ...), it
> is possible to upload the 'boot.scr' file to DRAM using the 'fel'
> tool. But U-Boot still needs to be able to pick it up there before
> checking any other boot media.
> 
> Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
> ---
> 
> The patch might be not the best way to implement this. But it would
> be great if U-Boot had out of the box support for:
>     http://linux-sunxi.org/index.php?title=FEL/USBBoot&oldid=13134#Boot_the_whole_system_over_USB_.28u-boot_.2B_kernel_.2B_initramfs.29
> 
> One of the bad things about this patch is that the "scriptaddr"
> variable needs to be hardcoded and protected agaist modifications
> (if this address is to be used from the SPL).

Perhaps use a separate dedicated/hardcoded address for the FEL boot
script to avoid adding unusual semantics to $scriptaddr (which might
surprise users not using fel)?

If the FEL address has to be 0x43100000 for compatibility with existing
instructions/tools that might mean moving the current scriptaddr
elsewhere. I think we can live with that.

> Also I'm not sure how this all could fit into the
> "config_distro_bootcmd.h" model, so I even have not tried
> that yet.

Just at a quick glance, based on the PXE entry something like:

#define BOOTENV_DEV_FEL(devtypeu, devtypel, instance) \
	"bootcmd_fel=source ...\0"

i.e. all the magic params are irrelevant in this case. Perhaps
"bootcmd_"#devtypel"=...\0" but I don't think that's needed in this
instance. BOOTENV_DEV_PXE doesn't bother at least.

Then in BOOT_TARGET_DEVICES include "func(FEL, fel, na)"

Ian.